The present invention is concerned with a circuit for use with electro-acoustic devices, such as sound-film devices, magnetic tape instruments, or similar devices with an audio amplifier connected to a signal source wherein an automatic control stage is used to control the amplifier's sensitivity.
In such devices, when the operating voltage is applied to the electronic circuits employed, asymmetric circuits, or unbalanced, unmatched components, can trigger voltage peaks in the amplifier that affect the automatic sensitivity adjustment of the amplifier adversely. Such automatic sensitivity adjustments are used in the independent tone control of many electro-acoustic devices in order to systematically achieve optimum conditions in modulating additional electronic devices when the input signals are of varying magnitude. For example, similar devices to control input sensitivity may be found in amplifying systems wherein signals are supplied by the pick-up device of a sound recorder to the output stage of the amplifier which is being used in order to maintain an audio level adequate for filling a room with sound and hence to provide an optimum sound recording.
Similarly, such circuits find significant use in the case of direct-sound motion picture cameras; wherein the adjustment of the sound signal to be recorded must be achieved instantly upon commencing operation. This is especially true in such case since, if a qualitatively unfavorable sound recording has been made it will hardly be possible to repeat the recording at a reasonable cost. Interferences occurring when the operation of the device is started can, in some cases, affect the automatic oscillator stage which is used to perform the adjustment of the sensitivity of the amplifier in such a way that the storage of the sound signals, particularly from the start of storage, is either to faint or too loud. Since the automatic sensitivity adjusting devices always have adjustment features determined by relatively large time constants, an adverse effect may occur; that is, from the start of a sound recording on, over a relatively long period of time, storage of the sound signals will be inadequate. Customarily, time constants in this type of adjusting devices are usually of such size that large input signals rapidly bring about a drop in the amplifier output level, and the sensitivity setting will then be maintained over a relatively long period of time.
It is therefore an object of the present invention to provide a circuit which avoids the drawbacks of known devices and does not require the user to handle any additional devices.